High temperature fuel cell system with integrated heat exchanger network

ABSTRACT

A fuel cell system ( 1 ) is provided and includes a fuel cell stack ( 3 ), a cathode recuperator heat exchanger ( 33 ) adapted to heat an air inlet stream using heat from a fuel cell stack cathode exhaust stream, and an air preheater heat exchanger ( 39 ) which is adapted to heat the air inlet stream using heat from a fuel cell stack anode exhaust stream.

BACKGROUND OF THE INVENTION

The present invention is generally directed to fuel cells and morespecifically to high temperature fuel cell systems and their operation.

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. High temperaturefuel cells include solid oxide and molten carbonate fuel cells. Thesefuel cells may operate using hydrogen and/or hydrocarbon fuels. Thereare classes of fuel cells, such as the solid oxide regenerative fuelcells, that also allow reversed operation, such that oxidized fuel canbe reduced back to unoxidized fuel using electrical energy as an input.

In a high temperature fuel cell system such as a solid oxide fuel cell(SOFC) system, an oxidizing flow is passed through the cathode side ofthe fuel cell while a fuel flow is passed through the anode side of thefuel cell. The oxidizing flow is typically air, while the fuel flow istypically a hydrogen-rich gas created by reforming a hydrocarbon fuelsource. The fuel cell, operating at a typical temperature between 750°C. and 950° C., enables the transport of negatively charged oxygen ionsfrom the cathode flow stream to the anode flow stream, where the ioncombines with either free hydrogen or hydrogen in a hydrocarbon moleculeto form water vapor and/or with carbon monoxide to form carbon dioxide.The excess electrons from the negatively charged ion are routed back tothe cathode side of the fuel cell through an electrical side of the fuelcell through an electrical circuit completed between anode and cathode,resulting in an electrical current flow through the circuit.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the invention, a fuel cell system is providedand includes a fuel cell stack, a cathode recuperator heat exchangeradapted to heat an air inlet stream using heat from a fuel cell stackcathode exhaust stream, and an air preheater heat exchanger which isadapted to heat the air inlet stream using heat from a fuel cell stackanode exhaust stream.

In one aspect, the air preheater heat exchanger is located upstream ofthe cathode recuperator heat exchanger, such that the air inlet streamis first heated by the anode exhaust stream followed by being heated bythe cathode exhaust stream prior to being provided into the fuel cellstack.

According to one aspect, the cathode recuperator heat exchanger isadapted to provide the cathode exhaust stream exiting the cathoderecuperator heat exchanger at a temperature of at least 200° C., thefuel cell stack and the cathode recuperator heat exchanger are locatedinside a hot box, and the air preheater heat exchanger and an air blowerwhich provides the air inlet stream are located outside the hot box.

In accordance with one aspect of the invention, a fuel cell systemincludes a fuel cell stack, a first means for heating an air inletstream using heat from a fuel cell stack cathode exhaust stream, and asecond means for heating the air inlet stream using heat from a fuelcell stack anode exhaust stream.

According to one aspect, the second means is located upstream of thefirst means, such that the air inlet stream is first heat by the anodeexhaust stream followed by being heated by the cathode exhaust streamprior to being provided into the fuel cell stack.

In accordance with one aspect of the invention, a method is provided foroperating a fuel cell system. The method includes the steps of heatingan air inlet stream being directed to a fuel cell stack using heat froma fuel cell stack anode exhaust stream, and heating the air inlet streamusing heat from a fuel cell stack cathode exhaust stream.

In one aspect, the air inlet stream is first heated by the anode exhauststream followed by being heated by the cathode exhaust stream prior tobeing provided into the fuel cell stack.

In accordance with one aspect of the invention, a fuel cell systemincludes a fuel cell stack, and a first means for heating an air inletstream using heat from a fuel cell stack cathode exhaust stream, whereinthe cathode exhaust stream has a temperature of at least 200° afterexiting the first means.

In a further aspect, the fuel cell system includes a second means forheating the air inlet stream using heat from a fuel cell stack anodeexhaust stream.

In yet a further aspect, the cathode exhaust stream has a temperature ofabout 200° C. to about 230° C. after exiting the first means, and thesecond means is located upstream of the first means, such that the airinlet stream is first heated by the anode exhaust stream followed bybeing heated by the cathode exhaust stream prior to being provided intothe fuel cell stack.

In accordance with one aspect of the invention, a method is provided foroperating a fuel cell system. The method includes the steps of providingan air inlet stream into a first heat exchanger, providing a fuel cellstack cathode exhaust stream into the first heat exchanger to heat theair inlet stream, wherein the cathode exhaust stream has a temperatureof at least 200° C. after exiting the first heat exchanger, andproviding the air inlet stream from the first heat exchanger into a fuelcell stack.

In a further aspect, the method further heating the air inlet streamusing heat from a fuel cell stack anode exhaust stream.

Other objects, features, and advantages of the invention will becomeapparent from a review of the entire specification, including theappended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of temperature versus heat for fluid flow in a systemof a comparative example.

FIGS. 2 and 3 are schematics of fuel cell systems according to the firstpreferred embodiment of the present invention. FIG. 2 is a systemcomponents and flow diagram and FIG. 3 shows the schematic of the heatexchanger network for the fuel cell system.

FIGS. 4, 5, 6 and 8 are plots of temperature versus heat for variousfluid flows in systems of the preferred embodiments of the presentinvention.

FIG. 7 shows the schematic of the heat exchanger network for the fuelcell system of the third preferred embodiment of the present invention.

FIG. 9 shows a somewhat diagrammatic representation of an integratedfuel humidifier assembly of the invention.

FIG. 10 is a somewhat diagrammatic representation illustrating the flowpaths of the assembly of FIG. 9.

FIG. 11 is a partially exploded perspective view of one embodiment ofthe assembly of FIG. 9.

FIG. 12 is a plan view of a heat exchanger plate of the assembly of FIG.11.

FIG. 13 is a partial, exploded perspective view of a heat exchangerplate pair for use in one embodiment of the assembly of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to maintain the SOFC at its elevated operating temperature, theanode and cathode flow streams exiting the fuel cell typically transferheat to the incoming flows through a series of recuperative heatexchangers. In a comparative example, this can include the process oftransferring heat to a liquid water source in order to generate steamfor steam reforming of a hydrocarbon fuel in order to generate thehydrogen-rich reformate flow.

For example, the cathode heat may be recuperatively transferred from thecathode exhaust flow stream to the incoming cathode air, while the anodeheat is partially recuperatively transferred from the anode exhaust tothe incoming humidified fuel, such as natural gas, which feeds the steamreformer, and partially transferred to the water to generate the watervapor being provided into the fuel to humidify the fuel. In addition,the water vapor within the anode exhaust may be recaptured to serveeither wholly or in part as the water source for the steam reformer.

The inventors discovered that a thermodynamic analysis of the system inwhich the anode (i.e., fuel side) exhaust stream is used to heat thehumidified fuel and to evaporate the water reveals that there will bemore energy available in the anode exhaust exiting the fuel cell than isrequired to be transferred to the incoming humidified fuel (i.e., waterand fuel). However, a sizable portion of both the heat available in theanode exhaust and the heat required for the feed is in the form oflatent heat. The result is that, while there is sufficient energyavailable in the anode exhaust, attempts to transfer the heat from theanode exhaust to the water and natural gas via a heat exchanger, inwhich the heat is transferred by convection from the anode exhauststream to a thermally conductive surface separating the exhaust streamand one or more of the incoming fluids, and from said surface to the oneor more of the incoming fluids, may not be commercially practical.

The above described problem is illustrated in FIG. 1, which shows theplot of temperature versus heat transferred for the anode exhaust andthe water. The conditions in FIG. 1 assume a 400° C. anode exhausttemperature entering an evaporator (i.e., vaporizer) from a water-gasshift reactor, and a hypothetical counter flow evaporator capable ofachieving full vaporization of the water, with minimal superheat.

As can be seen in FIG. 1, the condensing of water vapor from the fullysaturated anode exhaust and the isothermal vaporization of the watercauses the temperature of the heat rejecting anode exhaust to drop belowthe temperature of the heat receiving water for a substantial portion ofthe heat duty (i.e., the water curve is located above the anode exhaustcurve for Q values of about 1,100 to about 1750 W). As a result,achieving the required heat transfer between the fluids solely by use oftypical heat exchangers may not be feasible for the conditions assumedin FIG. 1, since the transfer of heat in a typical heat exchangerrequires the temperature of the thermally conductive separating materialto be less than the local bulk fluid temperature of the heat rejectingfluid, and higher than the local bulk fluid temperature of the heatreceiving fluid.

Therefore, an additional heating source may be needed to evaporatesufficient water to satisfy the amount of steam required for methanereformation, which can be as high as 1.5 kW in a system with 6.5 kWelectrical output. This additional heating source reduces systemefficiency.

The inventors discovered that the cathode (i.e., air side) exhaust maybe used to evaporate water being provided into the fuel and/or to heatthe fuel being provided into the system. By using this alternativeapproach to the recapture of heat energy in the SOFC fuel cell system,the entire thermodynamic potential of the exhaust gases can berecaptured for preheating of the fuel cell feeds without mass transferdevices such as an enthalpy wheel, or additional heat sources. However,in some systems utilizing this alternative approach, it still may bedesirable to utilize mass transfer devices such as an enthalpy wheel, oradditional heat sources. The system where the cathode exhaust is used tovaporize water for humidifying the fuel and/or used to heat incomingfuel is also be capable of being passively controlled. However, in somesystems where the cathode exhaust is used to vaporize water forhumidifying the fuel and/or used to heat incoming fuel, it may bedesirable to utilize active control.

FIGS. 2 and 3 illustrate a fuel cell system 1 according to a firstpreferred embodiment of the invention. Preferably, the system 1 is ahigh temperature fuel cell stack system, such as a solid oxide fuel cell(SOFC) system or a molten carbonate fuel cell system. The system 1 maybe a regenerative system, such as a solid oxide regenerative fuel cell(SORFC) system which operates in both fuel cell (i.e., discharge) andelectrolysis (i.e., charge) modes or it may be a non-regenerative systemwhich only operates in the fuel cell mode.

The system 1 contains one or more high temperature fuel cell stacks 3.The stack 3 may contain a plurality of SOFCs, SORFCs or molten carbonatefuel cells. Each fuel cell contains an electrolyte, an anode electrodeon one side of the electrolyte in an anode chamber, a cathode electrodeon the other side of the electrolyte in a cathode chamber, as well asother components, such as separator plates/electrical contacts, fuelcell housing and insulation. In a SOFC operating in the fuel cell mode,the oxidizer, such as air or oxygen gas, enters the cathode chamber,while the fuel, such as hydrogen or hydrocarbon fuel, enters the anodechamber. Any suitable fuel cell designs and component materials may beused.

The system 1 also contains a heat transfer device 5 labeled as a fuelhumidifier in FIG. 2. The device 5 is adapted to transfer heat from acathode exhaust of the fuel cell stack 3 to evaporate water to beprovided to the fuel inlet stream and to also mix the fuel inlet streamwith steam (i.e., the evaporated water). Preferably, the heat transferdevice 5 contains a water evaporator (i.e., vaporizer) 6 which isadapted to evaporate water using the heat from the cathode exhauststream. The evaporator 6 contains a first input 7 operatively connectedto a cathode exhaust outlet 9 of the fuel cell stack 3, a second input11 operatively connected to a water source 13, and a first output 15operatively connected to a fuel inlet 17 of the stack 3. The heattransfer device 5 also contains a fuel—steam mixer 8 which mixes thesteam or water vapor, provided into the mixer 8 from the first output 15of the evaporator 6 through conduit 10, and the input fuel, such asmethane or natural gas, provided from a fuel inlet 19, as shown in FIG.3.

The term “operatively connected” means that components which areoperatively connected may be directly or indirectly connected to eachother. For example, two components may be directly connected to eachother by a fluid (i.e., gas and/or liquid) conduit. Alternatively, twocomponents may be indirectly connected to each other such that a fluidstream passes between the first component to the second componentthrough one or more additional components of the system.

The system 1 also preferably contains a reformer 21 and a combustor 23.The reformer 21 is adapted to reform a hydrocarbon fuel to a hydrogencontaining reaction product and to provide the reaction product to thefuel cell stack 3. The combustor 23 is preferably thermally integratedwith the reformer 21 to provide heat to the reformer 21. The fuel cellstack 3 cathode exhaust outlet 9 is preferably operatively connected toan inlet 25 of the combustor 23. Furthermore, a hydrocarbon fuel source27 is also operatively connected to the combustor 23 inlet 25.

The hydrocarbon fuel reformer 21 may be any suitable device which iscapable of partially or wholly reforming a hydrocarbon fuel to form acarbon containing and free hydrogen containing fuel. For example, thefuel reformer 21 may be any suitable device which can reform ahydrocarbon gas into a gas mixture of free hydrogen and a carboncontaining gas. For example, the fuel reformer 21 may reform ahumidified biogas, such as natural gas, to form free hydrogen, carbonmonoxide, carbon dioxide, water vapor and optionally a residual amountof unreformed biogas by a steam methane reformation (SMR) reaction. Thefree hydrogen and carbon monoxide are then provided into the fuel inlet17 of the fuel cell stack 3. Preferably, the fuel reformer 21 isthermally integrated with the fuel cell stack 3 to support theendothermic reaction in the reformer 21 and to cool the stack 3. Theterm “thermally integrated” in this context means that the heat from thereaction in the fuel cell stack 3 drives the net endothermic fuelreformation in the fuel reformer 21. The fuel reformer 21 may bethermally integrated with the fuel cell stack 3 by placing the reformerand stack in the same hot box 37 and/or in thermal contact with eachother, or by providing a thermal conduit or thermally conductivematerial which connects the stack to the reformer.

The combustor 23 provides a supplemental heat to the reformer 21 tocarry out the SMR reaction during steady state operation. The combustor23 may be any suitable burner which is thermally integrated with thereformer 21. The combustor 23 receives the hydrocarbon fuel, such asnatural gas, and an oxidizer (i.e., air or other oxygen containing gas),such as the stack 3 cathode exhaust stream, through inlet 25. However,other sources of oxidizer besides the cathode exhaust stream may beprovided into the combustor. The fuel and the cathode exhaust stream(i.e., hot air) are combusted in the combustor to generate heat forheating the reformer 21. The combustor outlet 26 is operativelyconnected to the inlet 7 of the heat transfer device 5 to provide thecathode exhaust mixed with the combusted fuel components from thecombustor to the heat transfer device 5. While the illustrated system 1utilizes a cathode exhaust flow in the heat transfer device 5 that haspassed through a combustor, it may be desirable in some systems toutilize a cathode exhaust flow in the heat transfer device 5 that hasnot been passed through a combustor.

Preferably, the supplemental heat to the reformer 21 is provided fromboth the combustor 23 which is operating during steady state operationof the reformer (and not just during start-up) and from the cathode(i.e., air) exhaust stream of the stack 3. Most preferably, thecombustor 23 is in direct contact with the reformer 21, and the stack 3cathode exhaust is configured such that the cathode exhaust streamcontacts the reformer 21 and/or wraps around the reformer 21 tofacilitate additional heat transfer. This lowers the combustion heatrequirement for SMR.

Preferably, the reformer 21 is sandwiched between the combustor 23 andone or more stacks 3 to assist heat transfer. When no heat is requiredby the reformer, the combustor unit acts as a heat exchanger. Thus, thesame combustor 23 may be used in both start-up and steady-stateoperation of the system 1.

The system 1 also includes a fuel preheater heat exchanger (i.e., anoderecuperator) 29 which is adapted to heat the fuel inlet stream usingheat from the fuel cell stack 3 anode exhaust stream exiting from thestack 3 anode exhaust outlet 31. The system 1 further includes a cathoderecuperator heat exchanger 33 which is adapted to heat an air inletstream from an air blower 35 using heat from the cathode exhaust streamexiting the stack 3 cathode exhaust outlet 9. Preferably, the cathodeexhaust stream mixed with the combusted fuel components from combustor23 outlet 26 are provided into the cathode recuperator 33 to heat theair inlet stream. The cathode exhaust stream mixed with the combustedfuel components are then provided to the evaporator 6 of the heattransfer device 5 to evaporate the water to steam, which will then beprovided into the fuel inlet stream heading into the reformer 21.

Preferably, the fuel cell stack 3, the reformer 21, the combustor 23,the fuel preheater heat exchanger 29 and the cathode recuperator heatexchanger 33 are located in a hot box 37. Preferably, the cathoderecuperator heat exchanger 33 is intentionally undersized to ensure thatthe temperature of the cathode exhaust stream exiting the heat exchanger33 is sufficiently high to allow the heat transfer device 5 to evaporatethe water to steam via transfer of heat from the cathode exhaust stream.For example, in one highly preferred embodiment, the cathode recuperatorheat exchanger preferably has a size below a predetermined size, suchthat the cathode exhaust stream exits the cathode recuperator heatexchanger at a temperature of at least 200° C., such as 200° C. to 230°C., for example about 210° C. In this highly preferred embodiment, thecathode exhaust stream may enter the cathode recuperator heat exchanger33 at a temperature of at least 800° C., such as about 800° C. to about850° C., for example about 820° C. The cathode recuperator heatexchanger 33 is intentionally undersized to have an exchange rate ofabout 10 to 12 kW, such as about 11 kW for this highly preferredembodiment. In contrast, a full sized heat exchanger for the highlypreferred embodiment may have an exchange rate of about 16 kW. Whilespecific temperatures and heat exchange rates have been described forone highly preferred embodiment, it should be understood that the exitand entrance temperatures and heat exchange rates will be highlydependent upon the particular parameters of each specific application,and accordingly, it should be understood that no limitations to specificexit and entrance temperatures or heat exchange rates are intendedunless specifically recited in the claims.

The system 1 also preferably contains an air preheater heat exchanger 39which is adapted to preheat the air inlet stream from the air blower 35using heat from an anode exhaust stream exiting from the stack anodeoutlet 31. Preferably, the air blower provides an air inlet stream intothe system 1 which comprises at least 2.5 times, such as 2.5 to 6.5times, preferably 3 to 4.5 times as much air as required for the fuelcell stack 3 to generate electricity. For example, the blower 35 maypreheat the air inlet stream to about 50° C. The slightly preheatedinlet air stream is then provided from the blower into the air preheaterheat exchanger 39 where it is preheated to about 100° C. to about 150°C., such as about 140° C., for example. This preheated air inlet streamthen enters the cathode recuperator heat exchanger 33 at about 100° C.to about 150° C. and exits the heat exchanger 33 at about 700° C. toabout 750° C., such as about 720° C. Since the preheated air inletstream enters the cathode recuperator heat exchanger 33 at a temperatureabove room temperature, the cathode exhaust stream can exit the heatexchanger 33 at a temperature above 200° C. Thus, the air preheater heatexchanger 39 sufficiently preheats the air inlet stream to allow the useof an undersized cathode recuperator heat exchanger 33, which reducesthe overall system manufacturing cost.

Preferably, the air preheater 39 is located outside the hot box 37 andupstream of the cathode recuperator 33, such that the air inlet streamis first heated by the anode exhaust stream in the air preheater 39,followed by being heated by the cathode exhaust stream in the cathoderecuperator 33. Thus, the air inlet stream provided into the cathodeinlet 41 of the stack 3 is heated by both the anode and cathode exhauststreams from the stack 3.

The system 1 optionally contains a water gas shift reactor 43 which isadapted to convert at least a portion of water vapor in the fuel cellstack anode exhaust stream into free hydrogen. Thus, the inlet 45 of thereactor 43 is operatively connected to the stack anode outlet 31, andthe outlet 47 of the reactor 43 is operatively connected to an inlet 49of the air preheater 39. The water-gas shift reactor 43 may be anysuitable device which converts at least a portion of the water exitingthe fuel cell stack 3 fuel exhaust outlet 31 into free hydrogen. Forexample, the reactor 43 may comprise a tube or conduit containing acatalyst which converts some or all of the carbon monoxide and watervapor in the anode exhaust stream into carbon dioxide and hydrogen. Thecatalyst may be any suitable catalyst, such as an iron oxide or achromium promoted iron oxide catalyst.

The system 1 also optionally contains a condenser 51 adapted to condensewater vapor in the anode exhaust stream into liquid water, preferablyusing an ambient air flow as a heatsink. The system 1 also optionallycontains a hydrogen recovery system 53 adapted to recover hydrogen fromthe anode exhaust stream after the anode exhaust stream passes throughthe condenser 51. The hydrogen recovery system may be a pressure swingadsorption system or another suitable gas separation system, forexample. Preferably, the air preheater 39 partially condenses the watervapor in the anode exhaust stream prior to the anode exhaust streamentering the condenser 51 to reduce the load on the condenser 51. Thus,the outlet 55 of the air preheater 39 is operatively connected to theinlet 57 of the condenser 51. A first outlet 59 of the condenser 51provides hydrogen and other gases separated from the water to thehydrogen recovery system 53. A second outlet 61 of the condenser 51provides water to an optional water purification system 63. The waterfrom the purification system 63 is provided to the evaporator 6 whichcomprises a portion of the heat transfer device 5, through inlet 11.

The system 1 also optionally contains a desulfurizer 65 located in thepath of the fuel inlet stream from the fuel source 27. The desulfurizer65 removes some or all of the sulfur from the fuel inlet stream. Thedesulfurizer 65 preferably comprises the catalyst, such as Co-Mo orother suitable catalysts, which produces CH₄ and H₂S gases fromhydrogenated, sulfur containing natural gas fuel, and a sorbent bed,such as ZnO or other suitable materials, for removing the H₂S gas fromthe fuel inlet stream. Thus, a sulfur free or reduced sulfur hydrocarbonfuel, such as methane or natural gas, leaves the desulfurizer 65.

A method of operating the system 1 according to a first preferredembodiment of the present invention is described with reference to FIGS.2 and 3.

The air inlet stream is provided from the air blower 35 into the airpreheater 39 through conduit 101. The air inlet stream is preheated inthe air preheater 39 by exchanging heat with the anode exhaust streamcoming from the water-gas shift reactor 43. The preheated air inletstream is then provided into the cathode recuperator 33 through conduit103, where the air inlet stream is heated to a higher temperature byexchanging heat with the cathode exhaust stream. The air inlet stream isthen provided into the cathode inlet 41 of the stack 3 through conduit105.

The air then exits the stack 3 cathode outlet 9 as the cathode exhauststream. The cathode exhaust stream wraps around the reformer 21 andenters the combustion zone of the combustor 23 through conduit 107 andinlet 25. Desulfurized natural gas or another hydrocarbon fuel is alsosupplied from the fuel inlet 27 through conduit 109 into the combustor23 inlet 25 for additional heating. The exhaust stream from thecombustor 23 (i.e., cathode exhaust stream) then enters the cathoderecuperator through conduit 111 where it exchanges heat with theincoming air.

The cathode exhaust stream is then provided into the evaporator 6 of theheat transfer device 5 through conduit 113. The rest of the heat left inthe cathode exhaust stream is then extracted in the evaporator 6 forevaporating water for steam methane reformation before venting outthrough exhaust conduit 115.

On the fuel side, the hydrocarbon fuel inlet stream enters thedesulfurizer 65 from the fuel source 27, such as a gas tank or a valvednatural gas pipe. The desulfurized fuel inlet stream (i.e., desulfurizednatural gas) then enters the fuel mixer 8 of the heat transfer device 5through conduit 117. In the mixer 8, the fuel is mixed with purifiedsteam from the evaporator 6.

The steam/fuel mix is then provided into the fuel preheater 29 throughconduit 119. The steam/fuel mix is then heated by exchanging heat withthe anode exhaust stream in the fuel preheater 29 before entering thereformer through conduit 121. The reformate then enters the stack 3anode inlet 17 from the reformer 21 through conduit 123.

The stack anode exhaust stream exists the anode outlet 31 and isprovided into the fuel preheater 29 through conduit 125, where it heatsthe incoming fuel/steam mix. The anode exhaust stream from the hot box37 then enters the water gas shift reactor 43 through conduit 127. Theanode exhaust stream from reactor 43 is then provided into the airpreheater 39 through conduit 129, where it exchanges heat with the airinlet stream. The anode exhaust stream is then provided into thecondenser 51 through conduit 131, where water is removed from the anodeexhaust stream and recycled or discharged. For example, the water may beprovided into the water purifier 63 through conduit 133, from where itis provided into the evaporator through conduit 135. Alternatively,water may be provided into the purifier 63 through a water inlet 137,such as a water pipe. The hydrogen rich anode exhaust is then providedfrom the condenser 51 through conduit 139 into the hydrogen purificationsystem 53, where hydrogen is separated from the other gases in thestream. The other gases are purged through purge conduit 141 whilehydrogen is provided for other uses or storage through conduit 143.

Thus, as described above, the fluid streams in the system 1 exchangeheat in several different locations. The cathode exhaust stream iswrapped around the steam methane reformer 21 to supply the endothermicheat required for reformation. Then, natural gas or other hydrocarbonfuel is added directly to the cathode exhaust stream passing through thecombustor 23 as needed to satisfy the overall heat requirement forreformation. Heat from the high-temperature exhaust exiting thecombustor 23 (containing the cathode exhaust stream and the combustedfuel components, referred to as “cathode exhaust stream”) is recuperatedto the incoming cathode air (i.e., air inlet stream) in the cathoderecuperator 33. The heat from the anode exhaust stream exiting the anodeside of the fuel cell stack 3 is first recuperated to the incoming anodefeed (i.e., the fuel inlet stream) in the fuel preheater 29 and thenrecuperated to the incoming cathode feed (i.e., the air inlet stream) inthe air preheater 39.

Preferably, the air supplied to the fuel cell stack 3 from air blower 35is provided in excess of the stoichiometric amount required for fuelcell reactions, in order to cool the stack and take away the heatproduced by the stack. The typical ratio of air flow to stoichiometricamount is in excess of 4, such as 4.5 to 6, preferably about 5. Thisleads to substantially higher mass flow of cathode air than anode gas(i.e., fuel). Consequently, if the cathode exhaust stream only heats theair inlet stream, then the amount of heat which is transferred betweenthe cathode exhaust and air inlet streams is significantly higher thanthat which is transferred between the anode exhaust and fuel inletstreams, typically by a factor of approximately 3.

The inventors discovered that rather than transferring all of the heatwhich is recaptured from the cathode exhaust stream directly to theincoming air, the system 1 transfers only a portion of the cathodeexhaust stream heat to the incoming air inlet stream and uses theremainder of the available cathode exhaust stream heat for completevaporization of the water in the evaporator 6.

Thus, before the air inlet stream is heated to the appropriate fuel celltemperature, it is preheated by the anode exhaust stream in the airpreheater 39. This preheating ensures that the air inlet stream has asufficiently high temperature when entering the cathode recuperator 33to ensure that the recuperator 33 can raise the temperature of the airinlet stream to the appropriate fuel cell temperature

FIGS. 4 and 5 show graphs of the fluid temperature vs. the heattransferred for the evaporator 6 (i.e., the water vaporizer), and theair preheater 39, respectively, for one analyzed embodiment. As can beseen from the graphs in FIGS. 4 and 5, the thermodynamic cross-overshown in FIG. 1 is eliminated. This removes the need for either ahumidity exchanger or a supplemental heater which consumes additionalfuel.

In a heat exchanger, the “temperature approach” is defined as thesmallest temperature difference between the two fluid streams at anylocation in the heat exchanger. As can be seen in FIGS. 4 and 5, both ofthe heat exchangers (i.e., the evaporator 6 and the air preheater 39)have a very small temperature approach, located away from either end ofthe heat exchanger at the point where the two-phase region begins. It isadvantageous to maximize the temperature approach in each heatexchanger, since the rate of heat transfer between the fluids willdecrease as the local temperature difference between the streamsdecreases, leading to a need for a larger heat exchanger to transfer therequired heat.

If the portion of total cathode air preheat which occurs in the cathoderecuperator 33 is decreased, the temperature approach will increase inthe evaporator 6. However, the temperature approach will decrease in theair preheater 39. Conversely, if the portion of total cathode airpreheat which occurs in the cathode recuperator 33 is increased, thetemperature approach will increase in the air preheater 39. However, thetemperature approach will decrease in the evaporator 6. Of the totalcathode heat duty, there will then be some optimum percentage whichshould be transferred within the cathode recuperator 33 in order tomaximize the temperature approach in both the evaporator 6 and the airpreheater 39.

The inventors also discovered that by using the cathode exhaust streamfor vaporizing the water, the amount of superheat in the steam exitingthe evaporator 6 is very sensitive to the temperature and mass flow rateof the cathode exhaust stream entering the evaporator. This can be seenin FIG. 6, which shows the impact of a 4.5% increase in cathode exhauststream mass flow (with the cathode exhaust stream temperature into theevaporator remaining unchanged) on the resulting humidified natural gastemperature.

The temperature of the humidified natural gas entering the fuelpreheater 29 can be seen to increase by 28° C. due to this slightincrease in cathode exhaust stream flow rate. This increase intemperature will result in a higher anode exhaust stream temperatureexiting the fuel preheater, and subsequently a higher temperatureexiting the water gas shift reactor 43 and entering the air preheater39. This in turn leads to an increase in the cathode air preheat, whichwill tend to increase the temperature of the cathode exhaust streamentering the evaporator 6, thereby exacerbating the problem. Thehumidified natural gas temperature will continue to ratchet up,resulting in system stability problems, unless the inlet air flow rateis controlled. Thus, the cathode air (i.e., inlet air) flow rate needsto be controlled because it is one of the prime means of controlling thesystem 1.

In a second preferred embodiment, the previously mentioned potentialstability problems may be reduced or eliminated by having an adjustablecathode exhaust bypass around the evaporator 6, through which a smallportion of the cathode exhaust stream could be diverted in order tocontrol the cathode exhaust flow rate through the evaporator 6. Thissolution uses active control of the fluid flow rate.

In a third preferred embodiment, a passive approach is used to reduce oreliminate the previously mentioned potential stability problems withoutthe need for additional monitoring and control. The inventors havediscovered that a temperature of the humidified natural gas entering thefuel preheater 29 can be made to be relatively insensitive to changes inthe cathode exhaust stream flow rate and/or temperature by limiting thepotential for increased superheat in the evaporator through atemperature pinch.

FIG. 7 illustrates the heat exchanger portion of the system of the thirdpreferred embodiment. The other parts of the system of the thirdpreferred embodiment are the same as those of the first preferredembodiment shown in FIGS. 2 and 3.

As shown in FIG. 7, the direction of the water flow through theevaporator 6 is concurrent, rather than counter-current, with the flowof the cathode exhaust stream through the evaporator 6. Rather thanhaving the temperature approach in the evaporator 6 located at the onsetof the two-phase flow region, it is shifted to the end of the heattransfer region of the evaporator 6, where the temperature approach will“pinch” to a value of zero or closely approaching zero. No heat transferbetween the streams will occur after this point, and the two fluids willexit at or near a common temperature. The cathode exhaust stream flowrate may need to be increased slightly in order to ensure that the heatcapacity in the cathode exhaust stream is sufficient to achieve fullvapor quality in the water. The water (i.e., steam) will then exit theevaporator 6 with some amount of superheat. The cathode exhaust streamexiting the evaporator 6 can then be used to preheat the fuel, such asnatural gas in a second fuel preheater 67. Since the fuel inlet streamhas a very small flow rate compared to the cathode exhaust stream, it isquite easy to achieve 100% effective heat transfer and preheat the fuelinlet stream to the same temperature as the water vapor and cathodeexhaust stream exiting the evaporator.

Thus, as shown in FIG. 7, the system of the third preferred embodimentalso contains the second fuel preheater 67. The fuel preheater 67includes a first input 69 operatively connected to a cathode exhaustoutlet 9 of the fuel cell stack 3, a second input 71 operativelyconnected to the fuel source 27, and a first output 73 operativelyconnected to the fuel inlet conduit 17. The second fuel preheater 67 isadapted to transfer heat from the cathode exhaust stream of the fuelcell stack to the fuel inlet stream being provided to the fuel cellstack 3. The evaporator 6 in the third preferred embodiment comprises aconcurrent flow or “co-flow” evaporator in which the cathode exhauststream and the water are adapted to flow in a same direction, and anoutput of the evaporator is operatively connected to an inlet of thefuel preheater 67 such that the cathode exhaust stream flows from theevaporator 6 into the second fuel preheater 67.

Thus, the water and the cathode exhaust stream are preferably providedinto the same side of the evaporator and flow concurrent to each other.The water is converted to steam in the evaporator 6 and is provided intothe steam/fuel mixer 8. The cathode exhaust stream is provided from theevaporator into the second fuel preheater heat exchanger 67 where itheats the inlet fuel flow which is then provided through the mixer 8 andthe first fuel preheater heat exchanger (anode recuperator 29) into thestack 3.

The system of the third preferred embodiment is substantiallyinsensitive to variations in cathode exhaust stream temperature and massflow. FIG. 8 shows that, for one analyzed embodiment, the humidifiednatural gas temperature entering the anode recuperator (i.e., first fuelpreheater) 29 will increase by less than 7° C. due to a 6.8% increase incathode exhaust stream mass flow in the system of the third preferredembodiment. Such a small temperature rise should not cause thetemperature ratcheting described above, and therefore will result insystem stability without the need for active control of the inlet airand/or cathode exhaust stream flow.

Thus, in the preferred embodiments of the present invention, water isevaporated using the heat from cathode exhaust stream. The air heatexchanger (i.e., cathode recuperator) is undersized so that the hotstream exits it at a high temperature of at least 200° C., such as 200°C. to 230° C. Air is fed into the system at a stoic of 2.5 and above tohave enough exhaust heat for evaporating water needed for steam methanereformation. Preferably, between 2.5 and 6.5 times, more preferablybetween 3 and 4.5 times as much air is provided into the fuel cell stackas required for the fuel cell stack to generate electricity. The inletair entering the cathode recuperator is preheated in the air preheaterusing the anode exhaust stream to reduce the load on the cathoderecuperator. Water from the anode exhaust stream is partially condensedin the air pre-heater to reduce load in the anode condenser.

With reference to FIG. 9, the fuel humidifier 5 is preferably providedin the form of an integrated assembly 200 that includes, as a singleintegrated unit, the water evaporator 6, a fuel heater or preheater,such as the fuel preheater 67, and the fuel/steam mixer 8 connected toboth the water evaporator 6 to receive steam therefrom and the fuelheater 67 to receive heated fuel therefrom. The water evaporator 6preferably includes a water flow path 202 in heat transfer relation witha heat carrying fluid flow path 204, which in the illustrated system isa cathode exhaust gas flow path, while the fuel heater includes a fuelflow path 206 also in heat transfer relation with the heat carryingfluid flow path 204, which again is the cathode exhaust gas flow path204 for the illustrated system. The fuel/steam mixer 8 is connected toboth the water flow path 202 to receive steam therefrom and to the fuelflow path 206 to receive heated fuel therefrom. As seen in FIG. 9, thefuel preheater 67 is preferably located downstream from the waterevaporator 6 with respect to the heat carrying fluid flow path 204.However, in some applications, it may desirable for the fuel preheater67 to be located upstream from the water evaporator 6 with respect tothe heat carrying fluid flow path 204.

With reference to FIG. 10, in one preferred embodiment, the water flowpath 202 preferably includes a plurality of parallel water flow passages210, the fuel flow path 206 includes a plurality of parallel fuel flowpassages 212 and the heat carrying fluid flow path 204 includes aplurality of parallel heat carrying fluid flow passages 214 interleavedwith the water flow passages 210 in the water evaporator 6 andinterleaved with the fuel flow passages 212 in the fuel heater 67. Infurther reference to FIG. 10, the fuel/steam mixture 8 preferably is inthe form of a manifold or plenum 216 that is connected to all of thewater and fuel flow passages 210 and 212.

It is preferred that each of the water flow passages 210 include aliquid pressure drop inlet region 220 that provides a greater pressuredrop than the remainder 222 of the water flow passage 210 to help ensureproper distribution of the water flow to all of the water flow passages210. However, while the regions 220 are preferred, in some applicationsit may be desirable for the water flow passages 210 to be free of anysuch regions 220.

It is also preferred that each of the regions 220 be thermally isolatedfrom the heat carrying fluid flow path 206 by a thermal break, shownschematically at 224. The thermal break 224 acts to reduce conduction ofheat to the pressure drop inlet regions 220 and preferably prevents orlimits any vaporization of the water flow in the regions 220.

As seen in both FIGS. 9 and 10, the water flow and the heat carryingfluid flows have a concurrent flow relationship through the integratedassembly 200, the advantages of which were previously discussed hereinand which include providing stability for the associated system becauseof the temperature pinch and making the system less sensitive to changesin the flow rate of the heat carrying fluid, as well as temperaturechanges in the heat carrying fluid. While the concurrent flowarrangement is preferred, in some applications it may be desirable forthe flow to be arranged so as to provide a counter-current relationship,which can possibly allow for a lower flow rate and/or inlet temperaturefor the heat carrying fluid flow in comparison to the concurrent flowrelationship, or a higher humidified fuel outlet temperature.

FIG. 11 shows one preferred embodiment of the integrated fuel humidifierassembly 200. This embodiment utilizes a so-called stacked plateconstruction and includes a plurality of water/fuel plates or sheets 228interleaved with a plurality of heat carrying fluid plates or frames230, with each of the water/fuel plates defining one of the water flowpassages 210 and one of the fuel flow passages 212, and each of the heatcarrying fluid plates 230 defining one of the heat carrying fluidpassages 214.

Each of the water/fuel plates 228 further includes a water/fuel mixingchamber 232 that is open to both of the passages 210 and 21 2 to receivesteam and heated fuel, respectively, therefrom. Each of the heatcarrying fluid plates 230 also includes a water/fuel mixing chamber 234that is closed from the heat carrying fluid flow passage 214. Thechambers 232 and 234 are aligned to form the water/fuel mixing plenum216 that extends through all of the plates 228 and 230.

Each of the water/fuel plates 228 further includes a pair of heatcarrying fluid bypass openings 238 and 240 that are closed to thepassages 210 and 212 in the water/fuel plate 228. The openings 238 and240 in each of the plates 228 are aligned with the opposite ends,respectively, of the heat carrying fluid flow passages 214 in the heatcarrying fluid plates 230 to form a heat carrying fluid inlet manifold242 and a heat carrying fluid exit manifold 244, respectively, thatextend through all of the plates 228 and 230 to direct the heat carryingfluid into and out of, respectively, the passages 214.

Each of the water/fuel plates 228 also includes a water inlet opening246, with the openings 246 being aligned with each other and a waterbypass opening 250 in each of the heat carrying fluid plates 230 to forma water inlet manifold 252 that extends through all of the plates 228and 230.

Each of the heat carrying fluid plates includes a fuel bypass opening254, with the openings 254 aligned with an end of the fuel flow passage212 in each of the water/fuel plates 228 opposite from the chamber 232to form a fuel inlet plenum or manifold 256 that extends through all ofthe plates 228 and 230 to supply fuel to each of the passages 212.

The assembly 200 also includes separator sheets 260 that are interleavedbetween each of the plates 228 and 230 in order to seal their respectiveflow passages from each other, as is known in stacked plate heatexchanger constructions. Each of the separator sheets 260 has openings262, 264, 268, 270 and 272 that are aligned with and correspond to thechambers 232 and 234, the bypass openings 238, the bypass openings 240,the water inlet openings 246 and bypass openings 250, and the fuelbypass openings 254, respectively.

The assembly 200 also includes a pair of end plates 280 and 282 thatsandwich the plates 228 and 230 and sheets 260 to seal the assembly 200in a fluid tight manner. The end plate 280 includes a heat carryingfluid inlet connection or port 284 that is aligned with the heatcarrying fluid inlet manifold 242 to direct heat carrying fluid thereto,and a humidified fuel outlet connection or port 286 that is aligned withthe water/fuel mixing plenum 236 at an end of the plenum 236 oppositefrom the openings to the passages 210 and 212 to direct humidified fuelfrom aligned with the fuel manifold 256 to supply the fuel flow thereto,and a heat carrying fluid outlet connection or port 292 that is alignedwith the outlet manifold 244 to direct heat carrying fluid therefrom.

As best seen in FIG. 12, the passage 210 is defined by a continuous slotthat extends from the water inlet opening 246 to the water/fuel mixingchamber 232, with the slot being open to both faces of the plate 228.Similarly, the fuel passages 212 is defined by a continuous slot thatextends from the fuel inlet manifold 256 to the water/fuel mixingchamber 232, again with the slot being open to the opposing faces of thewater/fuel plate 228. With reference to both FIGS. 11 and 12, thepressure reduction region 220 of the passage 210 is defined by a portionof the slot that is formed in a tight serpentine pattern with arelatively narrow slot width, which together provide a tortuous flowpath. The water passage 210 then continues to a more open region of theslot where vaporization of the water occurs. In this regard, the initiallength of the slot adjacent the pressure reduction region 220 has areduced width in order to avoid separation of the water flow as it movesfrom the pressure reduction region 220 to the remainder 222 of the flowpassage 210, with the passage 210 widening further as it extends to thechamber 232.

As best seen in FIG. 12, each of the water/fuel plates 228 also includesthe thermal break 224 in the form of a slit or slot 300 that extends forthe length of the pressure drop inlet region 220 between the pressurereduction region 220 and the remainder 222 of the water flow passage210. As seen in FIG. 11, each of the heat carrying fluid plates 230includes a corresponding slit or slot 302, each of the separator sheets260 includes a corresponding slit or slot 304, and each of the endplates 282 includes a corresponding slit or slot 306, with all of theslits 300,302,304,306 being aligned throughout the stack to form aplenum 308 that extends through the stack and is open to atmosphere. Aspreviously discussed, the thermal break 224 acts to minimize conductionof heat to the pressure drop inlet region 220 and preferably prevents orlimits any vaporization of the water flow in the pressure reductionregion 220 to ensure that the water flow remains in the liquid phase inthe pressure reduction region 220. This is desirable because if thewater is allowed to evaporate, a high pressure drop could be produced inthe narrow passages of the pressure drop inlet region 220 and thatpressure drop could dominate. While the thermal break 224 is preferred,in some applications it may be desirable not to have the thermal break224 in the assembly 200.

As seen in both FIGS. 11 and 12, the flow passage 210 directs the waterflow in a globally concurrent flow relationship with the heat carryingfluid flow in the passage 214, but is formed with a serpentineconfiguration so as to provide localized cross flow with respect to theheat carrying fluid flow in the passage 214, thereby improving thetransfer of heat to the water while still providing the desiredconcurrent flow relationship.

Preferably, each of the flow passages 214 includes extended surfaces,which in the illustrated embodiment are shown in the form of a fin orturbulator insert 310, many suitable types of which are known. Extendedsurfaces may also be provided in the flow passages 210 and 212, but arenot shown in the illustrated embodiment.

With reference to FIG. 13, a water/fuel plate pair 312 is shown toillustrate one alternate embodiment for forming the water flow passage210. Each plate 314, 316 of the plate pair 312 includes a plurality ofdiscrete slots 318 that are arranged so as to overlie portions ofcorresponding discrete slots 318 in the opposite plate to form the waterflow passage 210, with the water flowing from one of the slots 318 inone of the plates 314, 316 to a corresponding slot 318 in the oppositeplate 314, 316 and then from that corresponding slot 318 back to asecond corresponding slot 318 in the first plate 314, 316 and so forthuntil the water flows into the water/fuel mixer 8. The pressurereduction region 220 in this embodiment is defined by multiple ones ofthe slots 318, each of a relatively narrow width and short length,thereby requiring multiple changes in flow direction and providing thetortuous flow path. For the particular arrangement of slots in FIG. 13,the water flow passage 210 is divided into three parallel legs 320, butit should be understood that such a configuration is optional and willbe highly dependent upon the requirements of each application. It shouldalso be appreciated that a plurality of the plate pairs 312 ofappropriate shape and size could be substituted for the water/fuelplates 228 in the embodiment shown in FIGS. 11 and 1 2.

While a couple of preferred embodiments for the assembly 200 have beenshown and described in connection with FIGS. 11-13, it should beunderstood that any suitable heat exchanger construction can be utilizedto form the assembly 200, including, for example, plate and bar typeconstructions, drawn cup constructions, nested plate constructions, andconstructions that incorporate discrete heat transfer tubes. It shouldalso be appreciated that the particular type of heat exchangerconstruction employed will be highly dependent upon the particularrequirements of the system in which the integrated humidifier assembly200 is employed. In this regard, it should be understood that while theintegrated fuel humidifier assembly 200 has been described herein inconnection with the fuel cell system 1, the integrated fuel humidifierassembly may find use in many other types of systems, and that nolimitation to a fuel cell system is intended unless expressly recited inthe claims.

While the integrated assembly 200 may be made utilizing any suitablematerial for the particular application, when employed in the fuel cellsystem 1 it is preferred that the sheets 260 and plates 228, 230, 280,and 282 be formed from stainless steel or another suitablecorrosion-resistant alloy and be nickel-brazed or brazed using anothersuitable corrosion-resistant brazing alloy.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

1. A fuel cell system, comprising: a fuel cell stack; a cathoderecuperator heat exchanger which is adapted to heat an air inlet streamusing heat from a fuel cell stack cathode exhaust stream; and an airpreheater heat exchanger which is adapted to heat the air inlet streamusing heat from a fuel cell stack anode exhaust stream.
 2. The system ofclaim 1, wherein the air preheater heat exchanger is located upstream ofthe cathode recuperator heat exchanger, such that the air inlet streamis first heated by the anode exhaust stream followed by being heated bythe cathode exhaust stream prior to being provided into the fuel cellstack.
 3. The system of claim 2, wherein: the cathode recuperator heatexchanger is adapted to provide the cathode exhaust stream exiting thecathode recuperator heat exchanger at a temperature of at least 200° C.;the fuel cell stack and the cathode recuperator heat exchanger arelocated inside a hot box; and the air preheater heat exchanger and anair blower which provides the air inlet stream are located outside thehot box.
 4. A fuel cell system, comprising: a fuel cell stack; a firstmeans for heating an air inlet stream using heat from a fuel cell stackcathode exhaust stream; and a second means for heating the air inletstream using heat from a fuel cell stack anode exhaust stream.
 5. Thesystem of claim 4, wherein the second means is located upstream of thefirst means, such that the air inlet stream is first heated by the anodeexhaust stream followed by being heated by the cathode exhaust streamprior to being provided into the fuel cell stack.
 6. The system of claim5, wherein: the first means is a means for providing the cathode exhauststream exiting the first means at a temperature of at least 200° C. thefuel cell stack and the first means are located inside a hot box; andthe second means is located outside the hot box.
 7. A method ofoperating a fuel cell system comprising: heating an air inlet streambeing directed to a fuel cell stack using heat from a fuel cell stackanode exhaust stream; and heating the air inlet stream using heat from afuel cell stack cathode exhaust stream.
 8. The method of claim 7,wherein the air inlet stream is first heated by the anode exhaust streamfollowed by being heated by the cathode exhaust stream prior to beingprovided into the fuel cell stack.
 9. The method of claim 8, wherein thecathode exhaust stream has a temperature of at least 200° C. aftereating the air inlet stream.
 10. A fuel cell system, comprising: a fuelcell stack; and a first means for heating an air inlet stream using heatfrom a fuel cell stack cathode exhaust stream, wherein the cathodeexhaust stream has a temperature of at least 200° C. after exiting thefirst means.
 11. The system of claim 10, further comprising a secondmeans for heating the air inlet stream using heat from a fuel cell stackanode exhaust stream.
 12. The system of claim 11, wherein: the cathodeexhaust stream has a temperature of about 200° C. to about 230° C. afterexiting the first means; and the second means is located upstream of thefirst means, such that the air inlet stream is first heated by the anodeexhaust stream followed by being heated by the cathode exhaust streamprior to being provided into the fuel cell stack.
 13. A method ofoperating a fuel cell system, comprising: providing an air inlet streaminto a first heat exchanger; providing a fuel cell stack cathode exhauststream into the first heat exchanger to heat the air inlet stream,wherein the cathode exhaust stream has a temperature of at least 200° C.after exiting the first heat exchanger; and providing the air inletstream from the first heat exchanger into a fuel cell stack.
 14. Themethod of claim 13, further comprising heating the air inlet streamusing heat from a fuel cell stack anode exhaust stream.
 15. The methodof claim 14, wherein: the cathode exhaust stream has a temperature ofabout 200° C. to about 230° C. afterexiting the first heat exchanger;and the air inlet stream is first heated by the anode exhaust streamfollowed by being heated by the cathode exhaust stream prior to beingprovided into the fuel cell stack.